08-09-03 New canopy transmissivity model

Introduced a new canopy transmissivity model (activated by the switch CanopyShade under Radiation Properties), which take into account the influence of solar elevation on the transmissivity of direct solar radiation through the canopy. To do this, the global radiation is firstly splitted into diffuse and direct fractions as a function of cloudiness (following Gryning et el, 1999) (this can be independently activated by another new switch SplitGlobal if canopy is not simulated), secondly, a transmissivity coefficient is estimated for direct and diffuse radiation separately, where the transmissivity of direct is dependent on solar elevation following Chen et al (2003), whereas the transmissivity of diffuse radiation (and longwave radiation) is calculated with the standard Beer's Law, independent of solar elevation. In addition, the sky view factor (1-{canopy soil cover fraction}) for direct radiation is also affected by the solar elevation as a consequence of the plant geometry represented by plant height and the (new) parameter PlantDiameter (also following Gryning et al, 1999).  Limitations: the new transmissivity model requires that the longwave radiation balance of the canopy is represented explicitly (see switch LongRadCanopy).

In addition, a correction was made on the estimation of cloudiness for sloped surfaces. Previously, the potential radiation was projected onto the slope before comparison with  the measured radiation to estimate cloudiness. However, in most cases the measured input is represents Global Radiation of a horizontal surface. Thus, in the new version, the potential radiation (if used as input to the model) is corrected for slope after the cloudiness estimation instead. Thus, simulation of cloudiness, and thereby incoming longwave radiation may change the model results to some extent.

Small differences compared to earlier versions may also occur in PotentialTranspiration and InterceptionEvaporation, due to the introduction of additional checks on the fraction of canopy above snow surface, to avoid any kind of evaporation or interception calculated for snow covered canopies. A new parameter is introduced (SnowReduceLAIThreshold), which sets the minimum fraction of canopy above snow surface to allow transpiration or interception evaporation.